In recent years, field effect transistors (FETs) using a nitride semiconductor such as gallium nitride (GaN) have been widely researched as power switching devices. Since GaN can form various compound materials with aluminum nitride (AlN) and indium nitride (InN), nitride semiconductors can form heterojunctions as do arsenic-based semiconductors such as gallium arsenide (GaAs). Therefore, they can be used for forming heterojunction field effect transistors (HFETs) with heterojunctions.
For most of devices currently used in the field of power electronics, normally-off type devices are strongly desired for ensuring safety in the event of a failure. However, even if a nitride semiconductor is undoped, spontaneous polarization and piezoelectric polarization produce a high concentration of carriers at the heterojunction interface. Therefore, when an FET is produced by using a nitride semiconductor, it is likely to be of the depression type (normally-on type), and it is difficult to obtain characteristics of the enhancement type (normally-off type) (see, for example, Patent Document 1).
Structures of normally-off type FETs using nitride semiconductors include, but not limited to: a structure in which the thickness and/or the Al composition ratio of an AlGaN layer being the barrier layer in the AlGaN/GaN heterojunction is reduced; a structure in which a recessed portion is provided in the gate portion to thereby shift the threshold voltage in the positive direction; a structure in which an FET is produced on the (10-12) plane of a sapphire substrate to prevent a polarization electric field from being generated in the crystal growth direction of the nitride semiconductor.
Junction field effect transistors (JFETs) having a p-type GaN layer formed in a gate portion have also been proposed (see, for example, Patent Document 2). A JFET employs, for the gate thereof, a pn junction having a larger built-in potential than that of a Schottky junction. Therefore, the gate turn-ON voltage can be increased, and the gate leakage current can be suppressed even when a positive gate voltage is applied.
Moreover, in the field of power electronics, there is a demand for a bidirectional switch capable of bidirectional current control, and a bidirectional switch using a GaN semiconductor has been proposed. (Patent Document 3)    Patent Document 1: Japanese Laid-Open Patent Publication No. 2004-273486    Patent Document 2: Japanese Laid-Open Patent Publication No. 2003-228320    Patent Document 3: United States Patent Application Publication No. 2005/189561